https://leetcode.cn/problems/invert-binary-tree/description/
翻转二叉树
给你一棵二叉树的根节点 root ，翻转这棵二叉树，并返回其根节点。



struct TreeNode* invertTree(struct TreeNode* root)
{
  if(root==NULL)
    return NULL;

  struct TreeNode *left=invertTree(root->left);
  struct TreeNode *right=invertTree(root->right);

  root->left=right;
  root->right=left;

  return root; 

}